1. Field of the Invention
This invention relates to an optical scanning apparatus for use in an image forming apparatus such as a laser beam printer or a digital copying machine.
2. Related Background Art
An optical scanning apparatus heretofore used in an image forming apparatus such as a laser beam printer or a digital copying machine directs a beam emitted from a light source to a deflecting element by incidence optical means, causes the beam deflected by the deflecting element to be imaged into a spot shape on a photosensitive drum surface which is a surface to be scanned by scanning optical means, and optically scans on the photosensitive drum surface with the beam.
In such an optical scanning apparatus, the beam emitted from the light source is converted into substantially parallel light beam by a collimator lens or the like, and in order to effect an optical face tangle error correction, the beam converted in substantially parallel light is formed into a linear image near a deflecting surface by a cylindrical lens. The photosensitive drum surface is scanned with the beam deflected by the deflecting surface of the deflector at a substantially constant speed by a scanning lens, and forms a spot.
In FIG. 9 of the accompanying drawings, the reference characters 1a, 1b, 1c and 1d designate light source means, each of which comprises, for example, a semiconductor laser. The reference characters 2a, 2b, 2c and 2d denote cylindrical lenses, each of which has predetermined refractive power in only a sub-scanning direction. Each of the light source means 1 and the cylindrical lenses 2 constitutes an element of incidence optical means.
The reference numeral 3 designates a light deflector as a deflecting element, and it comprises, for example, a rotating polygon mirror, and is rotated at a constant speed in the direction of arrow A by driving means (not shown) such as a motor. The reference numeral 11 denotes three fθ lenses having an fθ characteristic, and in a sub-scanning cross section, they bring the vicinity of the deflecting surfaces 3a and 3b of the light deflector 3 and the vicinity of photosensitive drum surfaces 100a, 100b, 100c and 100d as surfaces to be scanned into a conjugate relation to thereby have an optical face tangle error correcting function.
Also, in order to make the image forming apparatus compact, a return mirror for folding an optical path is disposed downstream of the deflector to thereby achieve compactness.
In the above-described conventional optical scanning apparatus, since a scanning lens is constituted by glass lens, usually an antireflection coat is provided on the surface of the lens and surface reflection does not occur, that is, it is difficult for an unevenness in light quantity on the surface to be scanned to occur.
The scanning lens is recent years, however, is constituted by a plastic lens and is designed to achieve a lower price and higher performance. In an optical scanning apparatus comprised of such a plastic-lens, it is generally difficult to provide an antireflection coat on the surface of the lens and therefore, there occurs unevenness in light quantity on the surface to be scanned caused by Fresnel reflection on the surface of the lens being changed by a scanning angle.
In order to solve these problems, as disclosed in Japanese Patent Application Laid-open No. H2-35413, it is known to set the reflectance of a return mirror so that reflectance near an optical axis and off-axis reflectance may differ from each other, and correct the unevenness in light quantity on the surface to be scanned. Also, as disclosed in Japanese Patent Application Laid-open No. H7-294837, there is known a technique of converting linearly polarized light into elliptically polarized light on this side of deflecting means, and correcting the unevenness in light quantity on the surface to be scanned, and as disclosed in U.S. Pat. No. 6,084,696, there is known a technique of changing the transmittance of an optical material to thereby correct the unevenness in light quantity on the surface to be scanned.
In an optical scanning apparatus described in Japanese Patent Application Laid-open No. H2-35413, however,
(1) consideration is given only to the reflecting characteristic depending on a polarizing direction,
(2) consideration is given only to the unevenness in light quantity due to the transmittance of the interior of the lens, and
(3) there is disclosed only a technique when a single return mirror is used in an optical path, and therefore, nothing is stated clearly about an unevenness in light quantity caused by surface reflection when the scanning lens is formed of plastics, and the correction of an unevenness in light quantity when there are two or more return mirrors in the optical path. Also, the technique disclosed in Japanese Patent Application Laid-open No. H7-294837 further requires optical members, and in the technique disclosed in U.S. Pat. No. 6,084,696, it is foreseen that the optical material is expensive, and by any of these techniques, it is difficult to construct an inexpensive optical scanning apparatus.